1 Field of the Invention
The present invention relates to an imaging apparatus, and more particularly, though not exclusively, to an imaging apparatus that picks up an image of a photographed object.
2. Description of the Related Art
Conventional imaging apparatuses, including digital cameras, use a CMOS sensor or a CCD as an imaging element. Especially in recent years, due to the demand for high performance digital cameras, the need for imaging elements having an increased number of pixels has increased.
FIG. 21 is a block diagram showing a conventional imaging apparatus.
A sensor 90 is driven by a timing generating apparatus (hereinafter, referred to as TG) 91. An AD converter 92 converts an output of the sensor 90 into a digital signal. The digital signal, as an image output, is entered into an image processing section 93. The image processing section 93 performs image processing and produces an image.
The sensors practically used for the imaging apparatuses are, for example, CMOS sensors and CCDs.
FIG. 15 shows a detailed arrangement of a conventional CMOS sensor.
A photodiode (hereinafter, referred to as PD) 51 is connected via a transfer switch 52 to a floating diffusion region (hereinafter, referred to as FD) 53. FD 53 is connected via a reset switch 54 to a reset voltage terminal 55. FD 53 is a gate of a field-effect transistor (hereinafter, referred to as FET) 56. A drain of FET 56 is connected to a predetermined voltage terminal. A source of FET 56 is connected via a selection switch 57 to a vertical output line 58. The above arrangement, minus the vertical output line 58, constitutes a pixel section 59. A plurality of pixel sections, disposed along the vertical output line 58, cooperatively form a column 60. Numerous columns cooperatively constitute the area sensor 90.
At least one constant-current source 61 is connected to each vertical output line 58. Electric charge in the floating diffusion region of a selected pixel section determines the voltage of the vertical output line 58. Furthermore, a capacitor 62 is connected via a changeover switch 63 to the vertical output line 58. The capacitor 62 functions as a memory that temporarily stores an output of the pixel section 59. Similarly, a capacitor 64 is connected via a changeover switch 65 to the vertical output line 58. The capacitor 64 functions as a memory that temporarily stores a reset level voltage.
The capacitor 62 is connected via the changeover switch 63 to a horizontal output line 66. The capacitor 64 is connected via the changeover switch 65 to a horizontal output line 67. The horizontal output lines 66 and 67 are connected to a differential amplifier 68. The differential amplifier 68 subtracts the reset level voltage stored in the capacitor 64 from the output voltage of the pixel section 59 stored in the capacitor 62. Thus, the differential amplifier 68 produces an image output.
FIG. 16 illustrates timing pulses produced from TG 91. TG 91 operates in the following manner. When a pixel section 59 is selected, a selection pulse becomes “High. The selection switch 57 is turned on.
In this case, a reset pulse is “High. The reset switch 54 is in the ON state. FD 53 is reset to a predetermined voltage. In this condition, a reset level voltage, i.e., a voltage obtained when FD 53 is maintained at a predetermined voltage, is output to the vertical output line 58. Subsequently, a memory pulse 1 becomes “High” and accordingly the changeover switch 65 shifts its movable contact to the vertical output line 58. The capacitor 64 stores the reset level voltage. Next, a transfer pulse becomes “High” and accordingly the transfer switch 52 is turned on. Electric charge of the pixel section 59 is passed (read out) from PD 51 to FD 53.
As FD 53 holds the transferred electric charge of the pixel section 59, FET 56 converts the electric charge into a voltage. The converted voltage is output to the vertical output line 58 as a voltage corresponding to the electric charge of pixel section 59. In this condition, a memory pulse 2 becomes “High” and accordingly the changeover switch 63 shifts its movable contact to the vertical output line 58. Thus, the capacitor 62 temporarily stores the voltage corresponding to the electric charge of pixel section 59.
Then, a read pulse becomes “High” to connect the changeover switches 63 and 65 to the horizontal output lines 66 and 67, respectively. The reset level voltage temporarily stored in the capacitor 64 is output to the horizontal output line 67. The voltage corresponding to the electric charge of pixel section 59 temporarily stored in the capacitor 62 is output to the horizontal output line 66. The differential amplifier 68 produces an output representing the output of the pixel section 59.
In the arrangement of FIG. 15, the capacitor 64, the changeover switch 65, and the horizontal output line 67 are the components used to remove the reset level voltage. However, these components can be omitted when the output of the pixel section 59 is directly taken out.
Furthermore, to improve signal quality of the CMOS sensor, FIG. 17 illustrates an arrangement for the CMOS sensor.
The pixel section shown in FIG. 17 is identical in arrangement with the above-described pixel section shown in FIG. 15, and accordingly will not be described in the following.
Furthermore, the vertical output line 58 and the constant-current source 61 are identical with those disclosed in the FIG. 15, and accordingly will not be described in the following.
A capacitor 70 is connected, at one end, to the vertical output line 58. The capacitor 70 temporarily stores the reset level voltage of FD 53. The other end of the capacitor 70 is connected to a positive input terminal of a differential amplifier 73. Furthermore, the other end of the capacitor 70 is connected to a switch 72 that is used for virtually grounding the differential amplifier 73, and is also connected to a capacitor 71 that temporarily stores a difference between the reset level voltage and a signal level voltage.
An output terminal of the differential amplifier 73 is connected via the changeover switch 63 to the capacitor 62 that functions as a memory temporarily storing the output of pixel section 59. Furthermore, the capacitor 62 is connected via the changeover switch 63 to the horizontal output line 66. Thus, the output of pixel section 59 can be read out of the capacitor 62 via an output amplifier 74 connected to the horizontal output line 66.
FIG. 18 shows timing pulses produced from TG 91. TG 91 operates in the following manner.
When a pixel section 59 is selected, a selection pulse becomes “High” and accordingly the selection switch 57 is turned on.
In this case, a reset pulse is “High. The reset switch 54 is in the ON state. FD 53 is reset to a predetermined voltage. In this condition, the reset level voltage, i.e., a voltage obtained when FD 53 is maintained at a predetermined voltage, is output to the vertical output line 58. Furthermore, a differential pulse is “High” in this condition. The switch 72 is closed and the reset level voltage is stored in the capacitor 70.
Then, the differential pulse turns into “Low. The switch 72 is opened, while the transfer pulse becomes “High. Accordingly, the transfer switch 52 is turned on. Electric charge of the pixel section 59 is passed (read out) from PD 51 to FD 53. As FD 53 holds the transferred electric charge of the pixel section 59, FET 56 converts the electric charge into a voltage. The converted voltage is output to the vertical output line 58 as a voltage corresponding to the electric charge of pixel section 59.
The capacitor 71 stores a difference voltage between the previous reset level voltage and the voltage corresponding to the electric charge of pixel section 59. The differential amplifier 73 produces an output voltage representing the difference. In this condition, a memory pulse becomes “High” and accordingly the changeover switch 63 shifts its movable contact to the vertical output line 58. Thus, the capacitor 62 temporarily stores the voltage corresponding to the electric charge of pixel section 59.
Then, a read pulse becomes “High” to connect the changeover switch 63 to the horizontal output line 66. The voltage corresponding to the electric charge of pixel section 59 temporarily stored in the capacitor 62 is output to the horizontal output line 66. The output amplifier 74 produces an output representing the output of the pixel section 59 (refer to Japanese Patent Application Laid-open No. 2002-252794).
An imaging element can be constructed from a CCD.
FIG. 19 illustrates a practical CCD arrangement.
PD 81 is connected via a transfer gate 82 to a vertical CCD 83. The vertical transfer CCDs can include a final stage CCD connected to a horizontal CCD 84. The horizontal transfer CCDs (e.g., including horizontal CCD 84) can include a final stage CCD connected to an output amplifier 85. The output amplifier 85 produces an image output.
FIG. 20 illustrates timing pulses produced from TG 91. TG 91 operates in the following manner.
When a transfer pulse becomes “High”, a photoelectron of PD 81 is transferred to a first stage of the vertical CCD 83.
The photoelectron transferred to the first stage of the vertical CCD 83 is successively transferred to succeeding stages of the vertical transfer CCDs in response to a V transfer pulse. Every time, the photoelectron is transferred by one step in the vertical transfer CCDs to the next, the photoelectron of the final stage of vertical transfer CCDs is transferred to a first stage of horizontal transfer CCDs (e.g., horizontal CCD 84). The photoelectron transferred to the first stage of the horizontal transfer CCDs is successively transferred to succeeding stages of the horizontal transfer CCDs in response to an H transfer pulse.
Every time, the photoelectron is transferred by one step in the horizontal transfer CCDs to the next, the photoelectron of the final stage of horizontal transfer CCDs is output to the output amplifier 85. The output amplifier 85 produces an image output.
However, according to the above-described conventional arrangement, the dynamic range of an imaging element is substantially dependent on the size of pixel sections. Thus, the conventional arrangement cannot increase the number of pixel sections in an imaging element.
Several years ago, the number of pixel sections constituting an imaging element was in a range from one million to two millions. However, the present imaging elements generally include five to six millions of pixel sections. Some of advanced imaging elements include tens of millions of pixel sections.
Digital cameras are very compact. Increasing the body size of an incorporated imaging element is substantially difficult and the pixel sections are decreased to a half size or less compared with those of several years ago. Accordingly, securing a satisfactory dynamic range for a sensor is difficult.